C04 EXTERNAL APPEARANCE OF GLACIERS. 
portions of the glacier advance at a much slower 
rate than its centre. 
The medial moraines arise in a different way, 
though they are directly connected with the lat- 
eral moraines. It often happens that two smaller 
glaciers unite, running into each other to form 
a larger one. Suppose two glaciers to be moving 
along two adjoining valleys, converging toward 
. each other, and running in an easterly or westerly 
direction ; at a certain point these two valleys 
open into a single valley, and here, of course, 
the two glaciers must meet, like two rivers rush- 
ing into a common bed. But as glaciers consist 
of a solid, and not a fluid, there will be no in- 
discriminate mingling of the two, and they will 
hold their course side by side. This being the 
case, the lateral moraine on the southern side of 
the northernmost glacier, and that on the northern 
side of the southernmost one, must meet in the 
centre of the combined glaciers. Such are the 
so-called medial moraines formed by the junction 
of two lateral ones. Sometimes a glacier may 
have a great number of tributaries, and in that 
case we may see several such moraines running 
in straight lines along its surface, all of which are 
called medial moraines in consequence of their 
origin midway between two combining glaciers. 
The glacier of the Aar, represented in the wood- 
cut opposite, affords a striking example of a large 
